International Concrete Abstracts Portal

With the increasing use of reinforced concrete for offshore structures, it is necessary to know the service behavior in addition to the ultimate behavior of beams to fatigue loading. An analytical method is presented here to predict the increase in deflection and crack width for concrete beams. The method is based on some available information on the fatigue properties of plain concrete, reinforced concrete beams and the ACI-design methodology for static loading. The analytical model is used to perform a parametric, and is compared with the CEB-FIP code equation and some available experimental data. The proposed method could be further improved as additional experimental data becomes available.

An account is given of eight fatigue tests on rein-orce concrete beams incorporating two types of mechanical splice, namely a single cold-forged sleeve and a screwed coupler connecting two cold-forged sleeves. The tests enabled the limiting range of bar stress to be found approximately, and the deflection and width of cracks were satisfactory under this range provided that the screwed coupler was fitted with a lock nut. In a comparative test, a beam with conventional splices consisting of straight lapped bars withstood the same fatigue stress as the cold-forged splices, but with increased deflection and maximum crack width. A second beam in which the lapped bars were cranked failed at a lower stress by fatigue fracture at one of the bends.

The few existing tests with stress reversals between tension and compression suggest that these may have little influence on the fatigue strength of concrete. This would instead be determined by the higher of the tensile or compressive stress maxima, expressed as percentages of the appropriate static strengths, with the minimum stress equal to zero. Two series of tests were performed to gain moreinformation. The specimens used were cubes and prisms loaded with compressive loads and transverse splitting line loads. Combination of these loads and pulsation of one of them gave the desired stress reversals. The test results indicate that stress reversals cause a slight reduction in fatigue strength. This reduction may however be due to the test equipment In the Swedish Code of Practice, a reduction of design stress due to fatigue caused by stress reversals between tension and compression is required. The results obtained are on the safe side of this requirement.

The objective of this investigation was to study ex-perimentally the behavior of plain concrete subjected to slow cyclical loading in compressive uniaxial and biaxial states of stress. A triaxial testing machine which had been successfully used to study the response of plain concrete cubicai specimens subjected to static multiaxial stress states was used in this study. The test specimens were subjected to prescribed stress histories which included 15 maximum stress levels. In addition to the uniaxial state of stress, biaxial stress states of two types were studied. The first was a proportional loading type in which two loading paths were used, namely 02/q = I .O and 02/01 = 0.5. The second loading type consisted of a constant stressing the direction with a cyclical loading in the 01 direction. The cyclical loading ranged from zero to various percentages of the unconfined static strength of the cube. Cycle rates used were in the range of one- cycle per minute. Strain measurements in all three principal directions were recorded for each test. This data was used to show the effects of number of cycles, load paths, and state of stress on the dilatational, as well as stress-strain response for plain concrete. The data waslso used to show the effect of state of stress and load path on the fatigue strength of concrete.

As part of an ongoing experimental/analytical research effort to evaluate the feasibility of a test method for fracture toughness of concrete, forty-eight plain concrete beams have been tested in bending to failure. All beams were notched and then precracked to different crack length/depth ratios-prior to load-ing to failure. The precracking was done using an electro-hydrodynamic materials testing system and displacement control. The beams which were cracked in fatigue were subjected to one million cycles of sinusoidal loading at 4 Hz. After the cycling was complete on a beam, the crack depth was determined using a compliance calibration technique following which the beam was loaded to failure. A load versus crack-mouth-opening-displacement trace was plotted during this final load run. For each beam tested in fatigue, a companion beam was pre-cracked "statically" by loading in repeated cycles until the crack depth, as measured by compliance calibration, matched that of the fatigued specimen. The studies were made on two different beam sizes in three and four-point bending with two different mix designs. Test results indicate the failure strength and associated maximum stress-intensity of the statically precracked beams to be slightly higher than those precracked in fatigue.